The Encrypted Collaboration Suite. CryptPad is the Zero Knowledge realtime collaborative editor. Encryption carried out in your web browser protects the data from the server, the cloud and the NSA. It relies on the ChainPad realtime engine.
Mind you, on their website they offer some premium features (https://cryptpad.fr/features.html), however if you install it on your own server, you can use it without trading any money, data, or attention to them.